A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    ⋆                      λ                          NA              PS                                                          (        1        )            where λ is the wavelength of the radiation used, NAPS is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways (either separately or in combination): by shortening the exposure wavelength λ, by increasing the numerical aperture NAPS, or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation system. EUV radiation systems are configured to output a radiation wavelength of about 13 nm. Thus, EUV radiation systems may constitute a significant step toward achieving small features printing. Such radiation is termed extreme ultraviolet or soft x-ray, and possible systems include, for example, laser-produced plasma sources, discharge-produced plasma sources, or synchrotron radiation from electron storage rings.
A laser-produced plasma source, when in operation, converts a material into a plasma state that has an element, e.g. Xe, Li or Sn with emission lines in the EUV range. The desired plasma can be produced by irradiating a target material, for example in the form of a droplet, stream or cluster of material, with a laser beam.
For this process, the plasma is typically produced in a sealed vessel, e.g., vacuum chamber, which may be referred to as plasma chamber, and monitored using various types of metrology equipment. In addition to generating EUV radiation, these plasma processes also typically generate undesirable by-products in the plasma chamber which can include out-of-band radiation, high energy ions and debris, e.g., atoms and/or clumps/microdroplets of the target material.
These plasma formation by-products can potentially heat, damage or reduce the operational efficiency of the various plasma chamber optical elements including, but not limited to, collector mirrors including multi-layer mirrors (MLM's) capable of EUV reflection at normal incidence and/or grazing incidence, the surfaces of metrology detectors, windows used to image the plasma formation process, and the laser input window. The heat, high energy ions and/or debris may be damaging to the optical elements in a number of ways, including coating them with materials which reduce radiation transmission, penetrating into them and possibly damaging structural integrity and/or optical properties, such as the ability of a mirror to reflect radiation at such short wavelengths, corroding or eroding them and/or diffusing into them. For some target materials, e.g., tin, it may be desirable to introduce an etchant such as a halogenide (e.g. HCl, Hl or HBr) into the plasma chamber. The etchant etches away material such as debris that has been deposited on the optical elements.
As indicated above, one technique to produce EUV radiation involves irradiating a target material. In this regard, CO2 lasers, e.g., outputting radiation at 10.6 μm wavelength, may present certain advantages as a laser irradiating the target material in a laser-produced plasma (LPP) process. This may be especially true for certain target materials, e.g., materials containing tin. For example, one potential advantage may include the ability to produce a relatively high conversion efficiency between the laser input power and the output EUV power. Another potential advantage of CO2 drive lasers may include the ability of the relatively long wavelength radiation (for example, as compared to deep UV at 198 nm) to reflect from relatively rough surfaces such as a reflective optic that has been coated with tin debris. This property of 10.6 μm radiation may allow reflective mirrors to be employed near the plasma for, for example, steering, focusing and/or adjusting the focal power of the drive laser beam. For 10.6 μm drive lasers, the window inputting the laser into the plasma chamber is typically made of ZnSe and coated with an anti-reflection coating. Unfortunately, these materials may be sensitive to certain etchants, e.g., halogenides.
In addition to the challenges presented by plasma generated debris, it can be difficult to consistently and accurately hit a series of moving droplets with a pulsed laser beam. For example, some high-volume EUV radiation sources may require the irradiation of droplets having a diameter of about 20-50 μm and moving at a velocity of about 50-100 m/s.
With the above in mind, systems and methods have been proposed for effectively delivering and focusing a laser beam to a selected location in an EUV radiation source.
U.S. Pat. No. 7,491,954 describes an EUV radiation source which comprises an optical gain medium and a lens which is arranged to direct radiation generated by the optical gain medium onto a droplet of fuel material. The optical gain medium and lens are arranged such that the optical gain medium generates laser radiation when the droplet of fuel material is at a predetermined location, thereby causing the droplet of fuel material to produce an EUV radiation emitting plasma. Since optical gain medium is triggered by the presence of the droplet of fuel material at the predetermined location, a seed laser is not required to trigger operation of the optical gain medium.